1. Field of the Invention
The present invention relates to a lithium silicate glass ceramic material and a process for fabricating that material for the manufacture of machinable blocks and subsequent fabrication of single crowns with the aid of a CAD/CAM device. The invention relates to a new version of such glass ceramic containing only lithium silicate as a main crystalline phase.
2. Background Art
There are many products available that employ lithium disilicate glass ceramic covered by several U.S. patents. Some of these patents claim a process for the preparation of shaped translucent lithium disilicate glass ceramic products from a mixture of basic components (SiO2, Al2O3, K2O, Li2O, plus pigments and fluorescent oxides). The thermodynamic solid-liquid equilibrium of the system consisting of lithium oxide (Li2O) and silicon dioxide (SiO2) has been extensively studied even before that material was used as a dental ceramic (1-3, 5-6).
For those skilled in the art this experimental solid-liquid equilibrium can help explain how different glass ceramics can be obtained using the same two components when they are combined in different proportions. The same solid-liquid equilibrium shows what type of stable crystal is produced as a final product of crystallization when a specific mix composition of the two components is blended, melted, and crystallized to achieve the final product.
The crystallographic data for intermediate crystal compounds in the Li2O—SiO2 system is given by the Landolt-Borntein tables (4). The following are the types of crystal compositions possible in the Li2O—SiO2 system: Li8SiO6, Li4SiO4 or lithium orthosilicate monoclinic and orthorhombic; Li6Si2O7, Li2SiO3 or lithium silicate; Li2Si2O or lithium disilicate monoclinic and orthorhombic; and Li2Si3O7 lithium trisilicate. Every crystal phase is formed based upon the initial molar ratio between the two components which can be 4, 3, 2 or 1 and any combination in between.
Thus when the silicon dioxide to lithium oxide molar ratio (SiO2/Li2O) is greater than or equal to two, meaning two moles of SiO2 are mixed with one mole of Li2O, the crystallized glass ceramic product will be mainly lithium disilicate (2SiO2.Li2O). This molar ratio of two is equivalent to a molar composition of lithium oxide in the binary mixture of 33% or 67% as SiO2. When the same molar ratio is below 2.0, (i.e. 1.7) only lithium silicate crystals are produced (Li2O.SiO2). The lithium oxide molar composition for a ratio of 1.7 is equivalent to 37% molar or 63% as SiO2. The type of resulting crystal due to the specific composition ratios gives to the glass ceramic its own distinguishable chemical and physical properties. Surprisingly, the same behavior is obtained if these two main components (silicon dioxide and lithium oxide) maintain their molar ratio below two even if they are mixed with other oxides as additives and modifiers. Such other common oxides are aluminum oxide, potassium oxide, calcium oxide, zirconium oxide and coloring oxides that are incorporated into the glass matrix and give the glass ceramic its final color and translucency.
Due to the final composition of this invention using a molar ratio of SiO2/Li2O between 1.7 to 1.9, the only phase present is lithium silicate, instead of lithium disilicate, as a main constituent of the glass ceramic as a final product. This lithium silicate (Li2O.SiO2) in this composition is thermodynamically stable meaning that it does not suffer any change in its stoichiometric compositor through the entire process. Once it is formed the only change suffered is the increase in size of the crystals at the end of the process. That means that there is no formation of any metastable lithium metasilicate which is converted to a lithium disilicate (2SiO2.Li2O) when the molar ratio (SiO2/Li2O) of the formulation is higher or equal to 2. For example, a glass ceramic with a molar ratio of silicon dioxide to lithium oxide greater than or equal to two plus additional oxides will produce, after full crystallization a lithium disilicate glass ceramic with a melting temperature of 920° C. and a linear thermal coefficient of expansion of 10.5×10−6/° C. as a final product and composition. In addition, during the production of this type of glass ceramic, the cast material is subjected to at least three different heat treatments: an annealing cycle for eliminating accumulated stresses, a nucleation cycle for the formation of lithium metasilicate or unstable lithium silicate, and finally a third thermal cycle to convert the unstable lithium silicate or metasilicate into a stable lithium disilicate. This complex mechanism is clearly shown in the following US patents:
Examples of those types of glass ceramics are claimed in Barret et al in U.S. Pat. No. 4,189,325 which discloses a lithium silicate glass ceramic where the raw materials are blended, melted at 1315° C. and held for 24 hours for homogenization, fritted and crushed, melted again and cast into preheated molds. They disclose a composition of silicon dioxide to lithium oxide molar ratio of two, producing a dental ceramic composed of lithium disilicate.
U.S. Pat. No. 4,480,044 to McAlinn discloses a glass ceramic formulation where the lithium silicate glass ceramic in their intermediate process stage has a thermal expansion of 13×10−6/° C. and the lithium disilicate has a thermal expansion of 11.4×10−6/° C. They disclose a machinable lithium disilicate glass ceramic with a percentage of silicon dioxide of 79.8%.
U.S. Pat. No. 4,515,634 to Wu et at discloses a castable glass ceramic composition useful as a dental restorative material. The components are blended and melted at 1400 to 1450° C., then quenched in water, dried, milled to a powder, and melted again at 1400° C. for 4 hours. Then the melt is cast into copper molds and transferred to the annealing process. The castable glass ceramic is lithium disilicate with a silicon dioxide to lithium oxide molar ratio of two, equivalent to silicon dioxide weight composition of 65%-74.7% and lithium oxide weight composition of 14.8-16.4%.
U.S. Pat. No. 5,219,799 to Beall et at discloses a lithium disilicate glass ceramic with silicon dioxide weight composition of 65%-80% and lithium oxide compositions of 8.0-19.0%. The blended raw materials are melted at 1450° C. for 16 hours and then poured into steel molds and annealed at 450° C.
U.S. Pat. No. 5,744,208 to Beall et at describes a lithium disilicate glass ceramic with silicon dioxide weight composition of 75%-95% and lithium oxide weight composition of 3-15%. The raw materials are blended, and then melted in the range of 1450-1600° C. for about 6-10 hours. The glass is then poured into steel molds. The glass is then annealed, nucleated and crystallized to produce lithium disilicate glass ceramic in the range of 500° C. to 850° C.
U.S. Pat. No. 5,968,856 to Scheweiger et al discloses a lithium disilicate glass ceramic with compositions of silicon dioxide weight between 57%-80% and lithium oxide composition 11-19%. The components are blended and melted at 1500° C. for one hour and then quenched, dried, milled, dry pressed and sintered to form blanks. The composition requires the addition of lanthanum oxide to improve the flow properties, control the crystal growth and eliminate the strong reaction of the material with the investment material used.
U.S. Pat. No. 6,514,893 to Scheweiger et al discloses a lithium disilicate glass ceramic with silicon dioxide composition of 57%-75% weight and lithium oxide composition 13-19% weight and also containing lanthanum oxide. The components are blended and fused into granulates and comminuted to a powder. Coloring oxides are then added, and the ceramic is pressed and heat treated.
U.S. Pat. No. 6,455,451 to Brodkin et al discloses a lithium disilicate glass ceramic with silicon dioxide composition of 62%-85% weight and lithium oxide composition 8-19% weight. They disclose a method of making the lithium disilicate by melting the components at 1200 to 1600° C., followed by quenching, drying, and heat treating to form the glass ceramic, followed by comminuting to a powder, compacting and sintering to a blank and pressing to form the restoration.
U.S. Pat. No. 6,517,623 to Brodkin et al discloses a pressable lithium disilicate glass ceramic where the components are melted in the range of 1200 to 1600° C., quenched, heat treated, comminuting the glass ceramic to a powder, and then compacting the powder to a starting blank before sintering the blank or the restoration.
U.S. Pat. No. 6,606,884 to Scheweiger et al describes a lithium disilicate glass ceramic where the components are mixed and melted at 1200 to 1650° C., followed by pouring the glass into water, milling and compacting, and placing the blank in a heat treatment to sinter.
U.S. Pat. No. 6,802,894 to Brodkin et al discloses a lithium disilicate glass ceramic with a silicon dioxide weight composition of 62%-85% and lithium oxide weight composition 8-19%. The components are mixed, melted at 1200 to 1600° C., and cast. The resulting glass is annealed at a range of 300 to 600° C., followed by subjecting the glass to a heat treatment from 400 to 1100° C.
U.S. Pat. No. 6,818,753 to Petticrew discloses a lithium disilicate glass ceramic with a silicon dioxide composition of 60%-80% weight and lithium oxide composition of 8-17% weight. The components are blended, melted, quenched, heat treated, milled to a powder, dry pressed, and hot pressed into the desired restoration.
Scheweiger et al U.S. Pat. No. 7,316,740 B2 claims a lithium silicate glass ceramic with silicon dioxide weight compositions of 64 to 73% and lithium oxide weight compositions of 13 to 17%. The lithium disilicate final product is demonstrated by means of a XRD pattern (FIG. 6) and DSC phase transformation curve from lithium metasilicate to lithium disilicate (FIG. 2). The DSC diagram shows the change in energy from the stage of metasilicate to disilicate, which is only necessary if lithium disilicate is desired to be the crystal phase used as a final product.
U.S. Pat. No. 7,452,836 to Apel et al discloses a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight producing lithium disilicate as a final product. They also describe a glass ceramic with a molar ratio of silicon dioxide to lithium oxide of at least 2.3.
U.S. Pat. No. 7,867,930 to Apel et al shows a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight producing lithium disilicate as a final product.
U.S. Pat. No. 7,871,948 to Apel et al describes a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight, producing lithium disilicate as a final product. The glass of the starting material is subjected to an initial heat treatment to form lithium metasilicate or unstable lithium silicate and then goes through a second heat treatment to convert the lithium metasilicate to a lithium disilicate.
U.S. Pat. No. 7,867,931 to Apel et al discloses a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight producing lithium disilicate as the final product. They also describe a glass ceramic with a molar ratio of silicon dioxide to lithium oxide in the range of 2.3 to 2.5.
U.S. Pat. No. 8,042,358 to Schweiger et al discloses a lithium silicate glass ceramic with silicon dioxide composition of 65%-70% weight and lithium oxide composition of 14-16% weight producing lithium disilicate as the final product. In their specific process the raw materials such as carbonates, oxides and phosphates are prepared and melted in the range of 1300-1600° C. for 2 to 10 hours. They explain that in order to obtain a particularly high degree of homogeneity, the glass melt obtained may be poured into water to form glass granulates and the glass granulates obtained are melted again.
U.S. Pat. No. 7,816,291 to Schwinger et al, discloses a lithium silicate dental glass ceramic where the starting glass uses a very specific composition and a specific process to provide in the intermediate stage a glass ceramic which consists of metastable lithium metasilicate. Then after machining the metastable lithium metasilicate, it is converted by two heat treatments into a lithium disilicate glass ceramic product with outstanding mechanical properties, excellent optical properties and very good chemical stability thereby undergoing only a very limited shrinkage. The metastable phase definition is broadly accepted by those skilled in the art as a thermodynamic unstable crystalline phase of lithium metasilicate which, when heated, disappears to allow the formation of a thermodynamic stable crystal phase form such as lithium disilicate. This is only possible for the specific composition used in this and all the prior art mentioned above.
For those skilled in the art it is well understood that the lithium oxide and silicon dioxide binary system has been extensively studied and several patents for dental glass ceramics have been granted in the last few years. However all the research so far falls in a range where lithium disilicate is formed as a final product and none of the references cited above discloses a lithium silicate glass ceramic as a final product. For those skilled in the art it is evident that the type of crystal produced depends exclusively on the molar ratio of silicon dioxide to lithium oxide in the glass ceramic and not the additives or modifiers added to the mixture. This molar ratio controls the type of crystal formed in the final composition and furthermore gives its name to the final glass ceramic.
More surprisingly, in the present invention and due to our specific formulation, there is no metastable lithium metasilicate in the intermediate stage or in the stage where the material can be easily machined into the shape of dental restorations. Instead a lithium silicate crystal which is thermodynamically stable is formed, keeping its stoichiometric composition through the entire process and therefore only grown as lithium silicate during the final stage of the process. As consequence no lithium disilicate is formed in the final stage of the process.
Most of the existing patents in the dental field describe the use the same basic components. The present invention uses germanium dioxide as a fundamental part of the formula. This oxide is broadly used in glass preparation for its good optical properties. The oxide has been well studied and has positive effects compared to common silicon glasses. It has been found that the addition of germanium oxide produces a melt with low viscosity, which facilitates the castability of the process and increases the thermal expansion and the refractive index of the resulting lithium silicate glass ceramic. More importantly, the addition of germanium dioxide increases the final density of the glass resulting in higher values of flexural strength than the lithium disilicate glasses free of germanium dioxide. U.S. Patent Application Publication No. 2004/0197738 to Ban et al discloses a process to make dental frame of zirconium-yttrium sintered ceramics and they describe dental porcelain with germanium oxide as a joint component different than the zirconium yttrium oxide frame. However germanium oxide is not used as a component of the framework ceramic network. It is used only in formulation of the ceramic joint and is just a part of a series of other oxides that can be joined to the framework material.
Due to the low silicon dioxide to lithium oxide molar ratio of 1.7 of the present invention, equivalent to 37% molar of lithium oxide (63% as silicon dioxide) the ceramic has a lower melting point compared to the glass ceramic of the prior art. In addition, this new glass ceramic contains the lowest silicon dioxide weight percent compared to all of the noted prior art. Therefore, due to this specific composition of lithium oxide in the mixture, the type of resulting crystal after crystallization (lithium silicate) gives to the glass ceramic its own chemical and physical properties, which makes it completely distinguishable from the prior glass ceramics noted above. Due to this distinguishable composition, the glass ceramic of the present invention, has a lower melting temperature which can be made even lower with the addition of germanium oxide. Germanium oxide replaces silicon dioxide in the glass network, causing it to have a negative effect on the resulting melting point compared to a glass ceramic containing only silicon dioxide. Thus the processing and optimal melting temperature is in the range of 1100° C. to 1200° C. instead of 1200° C. to 1650° C. of the U.S. patents cited above and specifically compared to U.S. Pat. No. 6,514,893 to Schweiger et al. The glass ceramics mentioned in the prior art patents cannot be cast in the range of 1100° C. to 1200° C. because they are too viscous due to their high silicon dioxide content therefore the processes disclosed in prior art patents with higher melting temperatures should be used. The present process will result in a more economical production because the energy employed for melting the glass is considerably lower and there are lower energy loses by radiation compared to the Schweiger process.
In addition to having a process with lower energy consumption, another significant improvement of the inventive process is related to the mixing and reaction of the components. In all of the cited prior art patents, the mix of the components is blended and melted at 1400 to 1650° C. and then cast or quenched in water. The quenched glass powder is dried, milled, and melted again in order to improve the homogeneity and the quality of the product. Surprisingly, we found that the first melting and casting process can be avoided if we perform a calcination process on the mixture of raw materials to a temperature in the range of 700 to 800° C. without melting the components. At this stage, all the raw materials in the form of salts (such as lithium carbonate as the source of lithium oxide, calcium carbonate as the source of calcium oxide, and di-ammonium phosphate as the source of phosphorous oxide) are decomposed, eliminating gases such carbon dioxide and ammonia, producing a ceramic powder free of gases. After cooling down, the calcined mix is milled again, producing a homogeneous powder with a very small particle size. The final step is melting and casting in the range of 1100° C. to 1200° C., resulting in a homogeneity of all the components. In addition, by eliminating the gases during the calcination process, the cast glass becomes bubble free, making this a significant advantage over the processes described in the prior art
The present invention is also unique compared to those in the prior art due to its composition. The use of a low melting temperature is only possible with the inventive glass ceramic because of the low content of silicon dioxide and the high content of lithium oxide. This translates to a molar oxide ratio (SiO2/Li2O) below 2.0, (i.e., 1.7) in which only lithium silicate crystals are produced (SiO2—Li2O). In addition to the composition, we have implemented a process for our glass ceramic that produces a homogeneous product and that can be used only with our specific formulation. This process cannot be used with the other prior art glass ceramics due to the lower operating temperatures.
It is emphasized that in the present glass ceramic the silicon dioxide and lithium oxide molar ratio content (SiO2/Li2O) is less than 2, specifically the oxide molar ratio is preferably about 1.7. This is specifically equivalent to 63% molar of silicon dioxide and 37% molar of lithium oxide, and specifically equivalent in the overall formulation of about 56% weight percent of all of the glass ceramic as silicon dioxide and 16.0% weight percent as lithium oxide and the remaining 28% composed of the oxide additives and modifiers. In all of the glass ceramic, only lithium silicate (Li2O.SiO2) crystals are produced as the final crystal phase product. During the heating process of the glass, the first crystals formed are stable lithium silicate and not metastable lithium metasilicate and such crystals remain stable through the end of the growing process. This means that there is no need for a third thermal process for producing the final crystal of lithium silicate making this an additional beneficial characteristic unique to the present invention. This new ceramic has a softening temperature of about 700 to 800° C. and a linear thermal coefficient of expansion of about 12 to 12.5×10−6/° C. as a final product and a composition yielding completely different chemical and physical properties compared to the prior art. This is easily demonstrated in commonly assigned U.S. patent application Ser. No. 12/1592,825, paragraph [0012], FIG. 1 showing a XRD pattern diffraction where only lithium silicate crystals are present in the final product and in paragraph [0013] thereof where the glass ceramic is shown to have a percentage linear change of 0.55% measured at 500° C. and an equivalent coefficient of thermal expansion of 11.5×10−6/° C.
The following is a list of non-patent references noted herein:                1. BOROM, P. et al, Strength And Microstructure In Lithium Disilicate Glass-Ceramics. The authors prepare lithium disilicate glass ceramics and measured the differences between the thermal expansion of the lithium disilicate with a value of 13×10−6/° C. and lithium silicate with a value of 11.4×10−6/° C. After the heat treatment above 800° C. the only phase present is lithium disilicate for a glass ceramic composition of 71.8% of silicon dioxide and 12.6% of lithium oxide.        2. EPPLER, A., Glass Formation And Recrystallization In The Lithium Metasilicate Region Of The System Li2O—Al2O3—SiO2 J. Am. Ceram. Soc., 46, {2}, 97-101, (1963).        3. HUMMEL, F. A., Thermal Expansion Properties Of Some Lithia Minerals, J. Am. Ceram. Soc., 34{8} 235-39. (1951).        4. LANDOLT-BÖRNSTEIN (LB), Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Technik.        5. S. CLAUS et al, Phase Equilibria In The Li4SiO4—Li2SiO3 Region Of The Pseudobinary Li2O—SiO2, Journal of Nuclear Materials, Volume 230, Issue 1, May 1996, Pages 8-11.        6. SHERMER, HERMAN, Thermal Expansion Of Binary Alkali Silicate Glasses, Journal of Research of the National Bureau of Standards. Vol. 57, No. 2, August 1956. The author prepares lithium silicate glasses with silicon oxide and lithium oxide molar ratio below 2.0 being lithium silicate with thermal expansion between 12 and 14.77×10−6/° C. There is no lithium disilicate using this chemical molar composition.        